Due to the nature of bank notes, passports, securities, and postage stamps, they are demanded to be less likely to be forged or altered, and aesthetic elements are also demanded. In order to meet these demands, engraving intaglio printing (hereinafter sometimes referred to simply as “intaglio printing”) that can provide excellent finishing of printed matters and involves a complicated plate making process that makes it difficult to forge printed matters is used for the printing of bank notes and the like.
Features of printed matters prepared by the intaglio printing method include unique hand feeling, the realization of formation of fine and sharp image lines, and the difficulty of producing the printed matter without use of special printing machines. For these reasons, intaglio printing is abundantly used for the production of the security printed matters.
The intaglio printing is a printing method that comprises preparing concaved image lines on a metallic printing plate, casting ink on the printing plate to fill the ink into concaves of the image lines, wiping off excess ink present on the intaglio printing plate, transferring the ink on a paper by applying a high pressure.
The maximum depth of concaves in the engraved plate generally reaches 100 μm. However, the ink transferred on the paper is a part of the casted ink, and the thickness of an ink film formed on the paper after the transfer is approximately 10 μm to 40 μm. The thickness of the ink film transferred on the paper is significantly larger than that of ink films obtained by other printing methods.
The intaglio printing ink used in the intaglio printing is produced by kneading a varnish ingredient, which comprises a drying oil-modified alkyd resin as a main component and a solvent and wax incorporated therein, with a pigment.
Further, in the intaglio printing method, as mentioned above, the thickness of ink films formed on printed matters is much larger than that of ink films formed by other printing methods such as offset printing. Accordingly, when printed matters formed by intaglio printing are carried in a mutually superimposed state, transfer of ink in a printed matter onto the backside of another printed matter superimposed on the printed matter, that is, a problem of set-off occurs. Various techniques are used for the prevention of the set-off and examples thereof include 1) a technique in which a slip sheet is inserted into between printed matters, 2) a technique in which a solvent is added to the ink to allow the solvent to be penetrated into the paper to accelerate the deposition of the resin ingredient in the varnish, and 3) a technique in which ink is allowed to stand at room temperature for hardening; in printing, the intaglio printing plate is heated to render the ink flowable and thus to facilitate the transfer of the ink; and, after printing, the flowability of the ink is lost and the adhesion of the ink is reduced.
Further, in the intaglio printing method, the step of wiping off excess ink on the intaglio printing plate is provided. The wiping-off step is carried out by a paper wiping method or a roll wiping method. From the viewpoints of the amount of wastes, printing speed and the like, a roll wiping method is mainly used in a large volume of printing.
The roll wiping method is a method in which excess ink present on the intaglio printing plate is wiped off by a wiping roller that is rotated in a direction opposite to the direction of rotation of a plate cylinder on which an intaglio printing plate has been mounted. The ink deposited on the wiping roller can be continuously wiped off by dispersing or dissolving the ink in an oleaginous solution or a surfactant-containing aqueous solution. The method using the oleaginous solution is called an oleaginous wiping method, and the method using the surfactant-containing aqueous solution is called an aqueous wiping method. The aqueous wiping method is mainly used from the viewpoint of a low load on working environments.
The intaglio printing ink can easily be wiped off by the roll wiping methods. Further, in order that the intaglio printing ink is dispersible or dissolvable in the surfactant-containing aqueous solution, an ink composition containing a varnish composed mainly of a low-viscosity and relatively low-molecular weight drying oil-modified resin is used. In the varnish composed mainly of the low-viscosity drying oil-modified resin, the intermolecular interaction of the varnish is so small that the varnish is likely to be penetrated into the printing paper and, thus, feathering or bleeding are observed in image lines by microscopic examination. This phenomenon leads to a new problem of lowered quality in fine image lines such as fine characters or the like used for forgery prevention purposes.
Increasing an ink drying speed for set-off prevention purposes causes the progress of a reaction for drying of the ink to some extent before ink transfer on the paper, sometimes leading to a failure of ink transfer.
Further, in the intaglio printing ink, the proportion of the varnish component used for ink fixation purposes is relatively lower than the proportion of the varnish component used other printing methods. Accordingly, a high-molecular weight resin component cannot be incorporated at a high mixing ratio in the varnish, and, thus, when image lines of the printed matter are rubbed by paper or the like, the paper is tainted. That is, a problem of chalking occurs. This poses a problem of contamination of a contact portion such as a roller or the like that delivers bank notes in machines that process bank notes at high speed, such as automatic bank note payment machines.
In offset printing in which the thickness of an ink film on paper is about 1 μm, from the viewpoints of preventing set-off of the printed matter and improving an anti-chalking property of the cured ink film, an ultraviolet drying method is extensively used that an ultraviolet curable composition is used as varnish and ultraviolet light is applied to cure the varnish. The application of the ultraviolet drying method to intaglio printing in which the thickness of an ink film on paper is 20 μm or more poses a problem that, when an ink having a high color density is used, the ultraviolet light cannot be permeated into the ink film, resulting in a failure of ink drying.
A drying method utilizing electron beams has been proposed as a method for curing a thick ink film that does not allow ultraviolet light to be permeated thereinto, and this method has been partially put into practical use. The curing method utilizing electron beams is disadvantageous in that an electron beam irradiation apparatus is expensive and nitrogen should be flown for the prevention of inhibition of polymerization by oxygen, leading to a high running cost. Accordingly, the percentage popularization is low. Further, the electron beams break molecules of the paper, and, thus, the strength of the paper is likely to be lowered.
The present applicant has proposed an ink having both ultraviolet curability and oxidation polymerizability and a printed matter using the ink with a view to curing a thick ink film that is not permeable to ultraviolet light (patent document 1). Patent document 1 describes that the use, as an intaglio printing ink, of a varnish comprising a mixture of an ultraviolet curable composition, an oxidation polymerizable composition, a photopolymerization initiator, and an oxidation polymerization catalyst can allow the surface of an ink film after printing to be cured with ultraviolet light to prevent set-off and, at the same time, can allow the interior of the ink film to be brought to a completely cured state by polymerization of the ultraviolet curable composition by radicals generated from the oxidation polymerizable material and the oxidation polymerization catalyst.
The present applicant has proposed a resin composition for an intaglio printing ink that can reduce the so-called blocking phenomenon, that is, a phenomenon that printed matters formed by intaglio printing are fixed to each other in the loading of the printed matters, by using polyglycerin polyacrylate having excellent flexibility and excellent ultraviolet curability as an ultraviolet curable composition, and an ink composition using the resin composition and a printed matter formed from the ink composition (patent document 2).
The present applicant has further proposed use of a solvent-free alkyd resin as an oxidation polymerizable composition (patent document 3).